qtmux: move subtitle layer above video and set alternate group
qtmux: move subtitle layer above video and set alternate group

layer -1 is above video, that is 0
And having all subtitles in alternate group 2 means that only one
should be selected at a time.

https://bugzilla.gnome.org/show_bug.cgi?id=737095

jcaden/gst-plugins-good,kittee/gst-plugins-good,GStreamer/gst-plugins-good,freedesktop-unofficial-mirror/gstreamer__gst-plugins-good,GStreamer/gst-plugins-good,pexip/gst-plugins-good,Kurento/gst-plugins-good,sh0/gst-plugins-good,sh0/gst-plugins-good,GStreamer/gst-plugins-good,shelsonjava/gst-plugins-good,freedesktop-unofficial-mirror/gstreamer__gst-plugins-good,ikonst/gst-plugins-good,BigBrother-International/gst-plugins-good,vatavuserban/gst-plugins-good,froggatt/gst-plugins-good-m,freedesktop-unofficial-mirror/gstreamer__gst-plugins-good,pexip/gst-plugins-good,froggatt/gst-plugins-good-m,sh0/gst-plugins-good,ijsf/OpenWebRTC-gst-plugins-good,ikonst/gst-plugins-good,rawoul/gst-plugins-good,pexip/gst-plugins-good,stfl/gst-plugins-good,jcaden/gst-plugins-good,chamois94/gst-plugins-good,sebras/gst-plugins-good,Kurento/gst-plugins-good,stfl/gst-plugins-good,jcaden/gst-plugins-good,ikonst/gst-plugins-good,GrokImageCompression/gst-plugins-good,shelsonjava/gst-plugins-good,vatavuserban/gst-plugins-good,loshca/gst-plugins-good,BigBrother-International/gst-plugins-good,BigBrother-International/gst-plugins-good,StreamUtils/gst-plugins-good,chamois94/gst-plugins-good,kittee/gst-plugins-good,Kurento/gst-plugins-good,hizukiayaka/gst-plugins-good,veo-labs/gst-plugins-good,ijsf/OpenWebRTC-gst-plugins-good,sebras/gst-plugins-good,jcaden/gst-plugins-good,stfl/gst-plugins-good,GrokImageCompression/gst-plugins-good,sebras/gst-plugins-good,kittee/gst-plugins-good,ijsf/OpenWebRTC-gst-plugins-good,StreamUtils/gst-plugins-good,rawoul/gst-plugins-good,ijsf/OpenWebRTC-gst-plugins-good,chamois94/gst-plugins-good,sebras/gst-plugins-good,chamois94/gst-plugins-good,rawoul/gst-plugins-good,hizukiayaka/gst-plugins-good,GStreamer/gst-plugins-good,GrokImageCompression/gst-plugins-good,veo-labs/gst-plugins-good,veo-labs/gst-plugins-good,loshca/gst-plugins-good,shelsonjava/gst-plugins-good,BigBrother-International/gst-plugins-good,froggatt/gst-plugins-good-m,kittee/gst-plugins-good,pexip/gst-plugins-good,vatavuserban/gst-plugins-good,stfl/gst-plugins-good,StreamUtils/gst-plugins-good,veo-labs/gst-plugins-good,vatavuserban/gst-plugins-good,hizukiayaka/gst-plugins-good,StreamUtils/gst-plugins-good,Kurento/gst-plugins-good,freedesktop-unofficial-mirror/gstreamer__gst-plugins-good,pexip/gst-plugins-good,sh0/gst-plugins-good,froggatt/gst-plugins-good-m,loshca/gst-plugins-good,rawoul/gst-plugins-good,ikonst/gst-plugins-good,shelsonjava/gst-plugins-good,hizukiayaka/gst-plugins-good,loshca/gst-plugins-good,GrokImageCompression/gst-plugins-good,Kurento/gst-plugins-good